1. Field of the Invention
The present invention relates to an optical disk apparatus for recording and reproducing information using a sample servo type optical disk, and an optical disk.
2. Description of the Related Art
A sample servo type optical disk has servo areas intermittently formed on a spiral tracks or concentrical tracks. In a recording/reproduction mode, signals necessary for clock reproduction, tracking, focusing and track accessing are extracted from a detection signal corresponding to the servo pattern.
In general, the servo area has wobbled pits formed on the right and left sides of a track with respect to its center, an access mark, a mirror portion, and a clock pit. The wobbled pits are formed to obtain a tracking error signal, the mirror portions are formed to obtain a focus error signal, and the clock pits are formed to reproduce a system clock in the recording/reproduction mode. The access marks form specific pit patterns which changes in units of tracks to detect the number of tracks (cross track count) which a light spot crosses when the light beam is relatively moved on the optical disk to access a track. In a track access mode, the access mark pattern is reproduced based on a detection signal corresponding to the access mark at every sample timing. The access mark pattern reproduced in synchronism with the present sample timing and the access mark pattern formed in synchronism with the previous sample timing are compared with each other to detect the cross track count between the two sample timings (the present sample period) together with information of a track cross direction. For example, when 16 different access marks are present, a cross track count up to 7 cross tracks can be detected in the direction.
A moving speed of the light spot relative to the optical disk is estimated from the cross track count. The cross track count at every sample timing is added to obtain a total cross track count during access or to measure a moving distance to a target track. In accordance with this measurement value, an aimed track access speed is selected. Thus, various control operations are available.
A track access operation is performed on the basis of data such as the cross track count, the cross direction, the relative moving speed of the light spot, the total cross track count, and the like.
In the track access mode, a detection signal can be obtained not only from tracks but also from a region between adjacent tracks although it has a low level. Since a light spot has a certain spot size, a noise component caused by crosstalk from adjacent tracks is generated in a detection signal from each track. In a track access mode, when a peak position of a detection signal from the access mark is shifted or a phase of a clock signal obtained from the clock pits is disordered due to a detection signal from the region between adjacent tracks or crosstalk of adjacent tracks, the access mark may be erroneously read as a different pattern.
When the access mark is read as a different pattern, it may be read as a pattern which is not present on the optical disk or as other pattern on the optical disk. In the former case, no cross track count data is obtained at all. However, in the latter case, a wrong cross track count is detected as if it were present.
As described above, in the conventional method, especially, when the light spot moves at high speed, the access mark for detecting a cross track count may be erroneously read due to a detected signal from a region between adjacent tracks or crosstalk from adjacent tracks. For this reason, it is difficult to correctly detect a cross track count, and a possibility of obtaining a wrong relative moving speed of a light beam estimated from the cross track count and wrong total cross track count data is increased, resulting in poor reliability of the track access operation.